It is well known that several organosulfur compounds are effective agents for increasing and stabilizing brightness in chemimechanical pulps and papers. Indeed a survey of the effects of molecular structure of a variety of S-containing compounds on their efficiacies in such bleaching and brightness stabilization has been conducted by Cole and Sarkanen [Tappi 72, 117 (1987); they concluded that optimum results were effected by certain monomeric thiols such as 1-thioglycerol and glycol dimercaptoacetate, and that disulfides, sulfoxides, sulfones and most thioethers were relatively ineffective agents in comparison.
It has been demonstrated that many of the reactions involved in the photoyellowing processes in chemimechanical and thermomechanical pulps and papers are free radical in nature, and in particular result from photochemical reactions of lignin macromolecules to generate free radicals. It has also been demonstrated that oxy and peroxy radicals are particularly damaging intermediates in photoyellowing processes; it is presumed that the enhanced bleaching and protection against reversion afforded by thiols results from their well documented free radical scavenging and facile hydrogen donating abilities. However, monomeric thiols such as those described by Cole and Sarkanen are expensive and possess prohibitively noxious odours. In addition, once the thiol group has reacted the agent is no longer an active antioxidant; thus relatively high charges (.about.5-7%) must be used.
In previous ESR studies both in solution with model lignin chromophores and in bleached and unbleached papers the relatively high effectiveness of thiolactic acid in prevention of brightness reversion has been observed. The synergistic effects of compounds such as ascorbic acid in prolonging the effectiveness of thiols as antioxidants by repair of the initially oxidized SH group are well known and similar to the synergistic effects of ascorbic acid and phenolic antioxidants such as Vitamin E.
Without wishing to be bound by this hypothesis, it is believed that the presence of an .alpha.-carbonyl function, such as that found in thiolactic or thioglycolic acid may provide a route for repair of the thiol function in an analogous fashion prolonging the effective lifetime of the antioxidant and minimizing the necessary quantity added.
In order to mitigate the odour problem associated with thiolactic or thioglycolic acid and other thiols it is now proposed by the present invention to graft thiolactate or thioglycolic groups onto the ends of a polymeric backbone so as to produce a compound which is odourless, transparent and soluble in water, alcohols, acetone and toluene.